CCIP_How to FE Analysis on Slab

8/6/2019 CCIP_How to FE Analysis on Slab

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Finite Element Analysis

FE AnalysisAdvantages

It assists in the design of slabs with complex

geometry where other methods require

conservative assumptions to be made.

It can be used to assess the forces around

large openings.

It can be used to estimate deflections

where other methods are time-consuming,

particularly for complex geometry. This

is provided that the advice on deflection

calculations later in this guide is followed.

It can be used for unusual loading conditions,

e.g. transfer slabs.

The model can be updated should changes

occur to the design of the structure.

Computer processing speeds are increasing;

reducing the time for alanysis.

Disadvantages

The model can take time to set-up, although

the latest generation of software has speeded

up this process considerably.

The redistribution of moments is not easily

achieved.

There is a steep learning curve for new users

and the modelling assumptions must be

understood.

Human errors can occur when creating the

model; these can be difficult to locate during

checking.

Design using FE requires engineering

judgement and a feel for the behaviour

of concrete.

IntroductionThe relative cost of computer hardware and software has reduced significantly

over recent years and many engineers now have access to powerful software

such as finite element (FE) analysis packages. However, there is no single

source of clear advice on how to correctly analyse and design using this type of

software. This guide seeks to introduce FE methods, explain how concrete can

be successfully modelled and how to interpret the results. It will also highlight

the benefits, some of the common pitfalls and give guidance on best practice.

What is FE and why use it?What is FE analysis?Finite element analysis is a powerful computer method of analysis that can be

used to obtain solutions to a wide range of one- two- and three-dimensional

structural problems involving the use of ordinary or partial differential

equations. For the majority of structural applications the displacement FE

method is used, where displacements are treated as unknown variables to be

solved by a series of algebraic equations. Each member within the structure

How to design reinforced concrete flat slabs using

Finite Element AnalysisO BrookerBEng, CEng, MICE, MIStructE

Prediction of slab deflection using an FE analysis program(courtesy of CSC (UK) Ltd).


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How to design reinforced concrete slabs using finite element analysis


to be analysed is broken into elements that have a finite size. For a

2D surface such as a flat slab, these elements are either triangular or

quadrilateral and are connected at nodes, which generally occur at the

corners of the elements, thus creating a mesh.

Parameters and analytical functions describe the behaviour of eachelement and are then used to generate a set of algebraic equations

describing the displacements at each node, which can then be solved.

The elements have a finite size and therefore the solution to these

equations is approximate; the smaller the element the closer the

approximation is to the true solution.

HistoryFE methods generate numerous complex equations that are too

complicated to be solved by hand; hence FE analysis was of interest

only to academics and mathematicians until computers became

available in the 1950s. FE methods were first applied to the design

of the fuselage of jet aircraft, but soon it was civil and structural

engineers who saw the potential for the design of complex structures.

The first application to plate structures was by R J Melosh in 1961 5.

Initially, the use of FE required the designer to define the location of

every node for each element by hand and then the data were entered

as code that could be understood by a computer program written

to solve the stiffness matrix. Nowadays this is often known as the

solver. The output was produced as text data only.

Many different solvers were developed, often by academic institutes.

During the 1980s and 1990s graphical user interfaces were developed,

which created the coded input files for the solver and then givegraphical representation of the results. The user interface that creates

the input files for the solver is often known as the pre-processor and

the results are manipulated and presented using a post-processor.

This has considerably simplified the process of creating the model and

interpreting the results. During the late 1990s and early 2000s the

software was enhanced to carry out design as well as analysis. Initially

the software post-processors would only calculate areas of reinforcing

steel required, but more recently the ability to carry out deflection

calculations using cracked section properties has been included in

some software.

When to use FE analysisA common myth is that FE will return lower bending moments and

deflections than would be obtained using traditional methods. Thisis a false assumption as, unless previous techniques were overly

conservative, it is unlikely that a different method of analysis would

give more favourable results. In fact a comparative study carried

out by Jones and Morrison6 demonstrated that using FE methods

for a rectangular grid gives similar results to other analysis methods

including yield line and equivalent frame analysis. Therefore, for simple

structures, there is no benefit in using FE analysis, and hand methods

or specialised software are probably more time-efficient.

FE analysis is particularly useful when the slab has a complex

geometry, large openings or for unusual loading situations. It may

also be useful where an estimate of deflection is required.

Initial sizingWhere FE is considered to be the correct tool for a project it will

generally be used only for detailed design. Initial sizing should still be

carried out using hand calculation methods such as:

Span-to-effective-depth ratios

Slab depths obtained from the publication Economic concrete

frame elements7 (see Table 1)

Previous experience

Using FE methods is unlikely to give a slab that is significantly thinnerthan when using simple hand methods.

AssumptionsIn preparing this guide a number of assumptions have been made to

avoid over-complication; the assumptions and their implications are

as follows.

Only flat soffits considered Only slabs with completely flat

soffits are considered in this guide. Where drop heads and beams

are also included in a model the following should be considered:

Table 1Economic depths (mm) for multiple span flat slabs

Imposedload

Span (m)

4 5 6 7 8 9 10 11 12

2.5 200 202 222 244 280 316 354 410 466

5.0 200 214 240 264 300 340 384 442 502

7.5 200 226 254 284 320 362 410 468 528

10.0 200 236 268 304 340 384 436 490 548

Assumptions

Class C28/35 concrete

Super-imposed dead load of 1.5 kN/m2

Perimeter load of 10 kN/m for cladding

Fire resistance 1 hour (increase depth by 10 mm for 2 hours)

Multiple spans (increase depth by 10 mm for 2 spans)

No holes


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Finite Element AnalysisFinite Element Analysis

l Most software will assume the centre of elements with

different thickness will be aligned in the vertical plane, so the

offset of the drop or beam should be defined in the model.

l The output is usually in the form of contour plots, and there

will be some interpretation required at the interface of

elements with different thicknesses.

The frame is braced It has been assumed that the lateral

stability is in the form of stability cores or alternative system

and that no additional moments are imposed on the column/

slab interface due to frame action. Where a stability frame is

used with a flat slab (recommended only for buildings with a

limited number of storeys) then the impact on the modelling

assumptions should be carefully considered. In particular,

where the horizontal forces are due to geometric imperfections

(notional horizontal loads), the long-term elastic modulus should

be used because these are long-term loads.

The concrete is not prestressed The guidance in this document

is not intended to be used for the design of post-tensioned

flat slabs.

Flat slab constructionDefinitionThe term flat slab has no universal definition. Eurocode 21 defines

flat slabs as slabs supported on columns. BS 81102 explicitly includes

waffle or coffered slabs. For the purpose of this guide, a flat slab is

considered to be a reinforced concrete slab of constant thickness,

which could include drop panels. However, this guide does not

specifically discuss how to model drop panels.

History of flat slabsThe flat slab was conceived as a structural system in the earliest days

of reinforced concrete development. Credit for inventing the flat

slab system is given to C A P Turner, and his system was described in

Engineering News in October 1905, and reviewed in a more recent

article3. Further development of the flat slab method was carried out

by Robert Maillart and Arthur Lord, and in 1930 the use of flat slabs

was codified in the 1930 London Building Act4.

Types of software availableIt is possible to model the whole building using a 3D frame analysis

package; the main advantages are that column stiffness can

automatically be included and that load takedowns are carried out.

However, the models become large and complex, requiring significant

computing power to solve the stiffness matrix as a complete model.

It is therefore preferable to carry out an analysis on a floor-by-floor

basis, either using a 3D package that allows this or by treating each

slab as an individual model.

Increasingly, FE packages have been adapted for particular uses (e.g.

reinforced concrete design) and many now include the ability to semi-

automate the design of the reinforcement as well as carry out the

analysis. Another feature that is almost standard is that CAD drawings

can be imported.

Although the software is now relatively simple to use, engineers

should still understand what the software is doing on their behalf

and what default parameters have been assumed in the package,

particularly for deflection calculations.

When selecting an FE software package it is important to understand

what it is capable of calculating. A list of features and their

importance are given in Table 2.

FE solvers can either use linear or non-linear analysis and the merits of

these are discussed below.

Linear analysisThis is currently the most widely used method of FE analysis, but it is

less sophisticated than non-linear analysis. Reinforced concrete (RC) is

treated as an elastic isotropic material, which it evidently is not,

and a number of assumptions have to be made to allow this

method to be used. These assumptions in the modelling can lead

to misunderstanding of the results and further explanation of

implications are discussed in the relevant sections throughout this

guide.

A linear analysis is more than adequate for carrying out a design atthe ultimate limit state. The serviceability limit state can be checked

by using deemed to satisfy span-to-effective-depth ratios or by using

conservative values for the elastic modulus and slab stiffness. Typically,

85% of elements are designed using the span-to-effective-depth

rules and this is considered to be perfectly adequate for the majority

of designs. Even the most sophisticated analysis will only give an

estimate of deflection in the range +15% to 30% .

Non-linear analysisMany FE packages are capable of carrying out non-linear (iterative)

analysis, but this is useful only for reinforced concrete design where itcan be used to model the cracked behaviour of concrete. Non-linear

analysis is used for RC design because as the slab is loaded it will crack

and this affects its stiffness. The program carries out an analysis with

uncracked section properties; it can then calculate where the slab has

cracked, adjust the material properties and run the analysis again. This

process continues until the variation in section properties between

runs reaches a predetermined tolerance.

A more sophisticated method is to also model the yielding of the

reinforcement where it reaches the elastic limit. This requires advanced

software and is generally used only for specialist situations; it is

outside the scope of this guide.


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Table Software features

Feature Benefit Is it required?

Features applicable for all types of FE analysis

The bending moments in orthogonal directions takeaccount of the torsion moment (e.g. are Wood Armermoments or similar methods included?)

Allows the design of the reinforcement to resist thefull design moments

Essential

Automatic mesh generation Saves time on creating the mesh. A good meshgenerator will save much time on refinements atcritical locations

No, but extremely useful

Columns and walls are entered as features in themodel and their stiffness is calculated by the software

This is a more efficient method than calculatingrotational spring supports by hand

No, but extremely useful

The area of the columns is automatically modelled asrelatively stiff elements by software

This will realistically reduce the deflections comparedwith a point support

No, but will give more realistic results for edgecolumns and will have economic benefits

Area of reinforcement calculated by the software Enables contour plots to be generated showing areasof steel as well as bending moments

No, but useful

Software analyses in-plane slab forces and considersvariations in slab centroid elevation

Allows realistic analysis of slabs with varyingthicknesses

If slab is not of uniform thickness (unless slabcentroid elevation is uniform) or contains beams

Automatic application of load patterns to determineworst case design forces

Ensures the worst combinations of forcesare obtained

No, the worst credible load arrangements can befound using a limited number of load patterns

Features applicable where estimated deflections are required

Curvature due to free shrinkage strain calculated A requirement of BS 8110 and Eurocode 2 fordetermining deflections

Yes, where estimated deflections are required

Cracked section properties calculated for everyelement and recalculated for subsequent iterations

Cracked section properties vary throughout the slab Yes, where estimated deflections are required

Cracked section properties calculated in each direction Cracked section properties vary in each direction Yes, where estimated deflections are required

Partially cracked properties are calculated Tensioning stiffening will prevent a fully crackedsituation


Separate analysis used for ULS and SLS Less cracking occurs at the SLS, so the slab ismore stiff


Software calculates creep coefficients, tensile strengthand free shrinkage strains for each change in loadingthroughout the life of the slab.

Saves calculating by hand No

Proposed reinforcement arrangements can be appliedto the model

The size and distribution of the bars affects thecracking and crack patterns


Features applicable for design using FE software

Areas of required reinforcement can be averaged overa specified width

This automation saves time No, but useful

FE analysis anddesign procedure

A recommended process of design using FE analysis is given in Figure 1,and commentary is provided below.

What results are to be expected?Before any analysis is carried out using computer software it is always

good practice to carry out some simple hand calculations that can

be used to verify that the results are reasonable. It is particularly

important to do this when using FE, and not treat the computer as a

black box. Simple calculations can be carried out to determine the

free bending moment, i.e. calculate wL2/8 for a span and then check

that the FE results give the same value between the peak hogging

and sagging moments. A discrepancy of 20% is acceptable; outside ofthis limit further investigation should be carried out to determine the

reasons. Calculate the total load on the slab and compare these against

the sum of the reactions from the model. Always include any hand

checks in your calculations.


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AnalysisHaving carried out the initial sizing and calculated the expected

magnitude of the results an FE model can be created. The initial

results should be used to determine the ultimate limit state (ULS)

requirements. From these results a preliminary bar size and layout can

be determined. These are required in order to determine the stiffness ofthe slab, which is essential for checking the serviceability criteria.

Check serviceability criteriaAfter determining the slab stiffness and the elastic modulus, the

estimated deflection can be calculated using this data in the FE model,

and checked against acceptance criteria.

Additional reinforcement may be added in the mid-span to control

deflection, but it is important to remember that this will increase the

stiffness in the middle of the slab. Therefore the model should be

re-analysed and the ULS checked again. Note that where the span-to-

START

Use hand methods to determine slab depth

Carry out hand calculations to verify results to be obtained from the FE analysis

Figure 1Design process using FE analysis

Yes

Non-linear analysis. Initially assume As,reqd = As, provLinear analysis?No

effective-depth ratios from Eurocode 2 are applied the UK National Annex

allows only 50% extra reinforcement to be used for deflection control.

Governing criteriaPunching shear and deflection control are usually the governing criteria

for flat slabs. Punching shear should be checked using code rules.

Deflection in concrete is a complex phenomenon, which is dependent

on the final tensile and compressive strength, elastic modulus,

shrinkage, creep, ambient conditions, restraint, loading, time and

duration of loading, and cracking of the member (see Panel 1). Many of

these factors are inter-related and often difficult to assess. Deflection

prediction is based on assumptions and is therefore an estimate even

when using the most sophisticated computer software.

Importantly, deflection in a reinforced concrete slab is dependant on

the age at first loading and the duration of the load because it will

1. Use long term elastic modulusELT= EST/6 for storage & plant loads &ELT = EST/4 for ofce & residential loadswhere EST = short term elastic modulus

EST can be obtained from Table 7.2 of BS 8110 Pt 2 orTable 3.1 of BS EN 1992-1-1.

2. Alternatively check serviceability using span-to-effective-depth ratios.

Calculate the tensile strength and creep coefcients

Carry out verication checks

Determine area of steel required at ultimate limit state

Check deection.1. Rene ELT if necessary and re-run analysis, or

2. Increase area of mid-span bottom reinforcement as requiredto meet the span-to-effective-depth ratios.

Check transfer moments at edge and corner columns

Check punching shear

FINISH

Create model and run iterative cracked section analysis.A stiffness matrix is required for both ULS and SLS

Check deections are reasonable

Determine preliminary reinforcement layout and apply to model

Run analysis again for both SLS and ULS

Check deections and stress in reinforcement revise modeland run analysis again if necessary

Create model and run analysis


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Codes, the yield line or grillage methods can also be used. (subject to

Cl 9.4 of Eurocode 2-1-1).

Some engineers are inclined to believe that by using FE analysis the

Code requirements do not apply; in particular they consider that there

is no need to check the maximum permissible transfer momentsbetween the slab and column. However, it needs to be understood that

FE is an elastic method, just like the elastic frame method described in

the Codes, and the provisions of Eurocode 2 Annex I.1.2(5) or BS 8110

Cl.3.7.4.2 and 3.7.4.3 should still be applied.

Creating an FE modelProperties of concreteReinforced concrete is a complex material, consisting of reinforcing

steel, aggregates, water, cementious material, admixtures, and probably

voids and un-hydrated cement. The properties of concrete are affected

significantly by the different types of aggregate and by the varying

proportions of the constituent materials. The properties of concrete

are also affected by workmanship, weather, curing conditions and age

of loading.

Both BS 8110 and Eurocode 2 allow reinforced concrete to be

modelled as an elastic isotropic material. Clearly this requires a number

of assumptions to be made and the limitations of these assumptions

should be fully understood by the designer. The impact of these

assumptions will be discussed later in this guide. The deflection ofthe slab is mainly dependant on tensile strength, creep and elastic

modulus.

influence the point at which the slab has cracked (if at all) and is used to

calculate the creep factors. A typical loading sequence is shown in Figure 2,

which shows that in the early stages relatively high loads are imposed

immediately after casting the slab above. Once a slab has cracked it will

remain cracked and the stiffness is permanently reduced.

Methods of analysis and code requirementsFE is not the only method for analysing flat slabs. In addition to

the tabular method and elastic frame methods described in the

What affects deflection?

There are numerous factors that affect deflection. These

factors are also often time-related and interdependent, which

makes the prediction of deflection difficult.

The main factors are:

Concrete tensile strength

Creep

Elastic modulus

Other factors include:

Degree of restraint

Magnitude of loading

Time of loading

Duration of loading

Cracking of the concrete

Shrinkage

Ambient conditions Secondary load-paths

Stiffening by other elements

Figure Loading history for a slab

0

2

4

6

8

10

12

14

0 50 100 150 200 250 300

Duration (days)

Load(kN/m) a

b

c

de

f

gh

Loading sequence

a Slab struck

b 1st slab above cast

c 2nd slab above cast

d 3rd slab above cast

e Floor finishes applied

f

g Quasi-permanent variable actions

h Frequent variable actions

Partitions erected


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design of the ULS only, the elastic modulus is not usually critical

because the results should always be in equilibrium.

Poissons ratio A value of 0.2 should be used for Poissons ratio.

Element typesWhen carrying out FE analysis, the selection of a particular type

of element is no longer necessary as most commercially available

software packages for flat slab design do not offer an option. For

reference it is usual to use a plate element; this will provide results for

flexure, shear and displacement. In the future it is likely that membrane

action will be modelled and considered in the design, in which case a

shell element would be used.

Plate and shell elements are generally triangular or quadrilateral with

a node at each corner (see Figure 3). However, elements have been

developed that include an additional node on each side, this gives

triangle elements with six nodes and quadrilateral elements with eightnodes. Since the only places where the forces are accurately calculated

are at the nodes (they are interpolated at other positions), the accuracy

of the model is directly related to the number of nodes. By introducing

more nodes into an element the accuracy of the results is increased;

alternatively, the number of elements can be reduced for the same

number of nodes, so reducing computational time.

Where the slab is deep in relation to its span (span-to-depth


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there are insufficient nodes and the results are based on interpolations

between the nodes. However, a very fine mesh will take an excessive

time to compute, and is subject to the law of diminishing returns.

The importance of selecting the correct mesh size is illustrated in

Figure 5. The same model was analysed three times with the only

change being the maximum mesh size. Where a very coarse mesh was

used (up to 5000 mm) it took just 30 seconds to analyse; although

it is analytically correct it does not give sufficient detail. Conversely,

when a much finer mesh was used (up to 500 mm) it took 15 minutes

to analyse and gives the shape of bending moment diagram that

would be expected. However, a mesh up to 1000 mm took just four

minutes to analyse; it gave very similar results and is considered to be

sufficiently accurate for the purpose of structural design.

As the processing speed of computers increases there will be less need

to be concerned about optimising the mesh size; but it is worth notingthat, although the 500 mm mesh gave notionally more accurate

results, the reinforcement provision would have been identical for both

the 500 and 1000 mm mesh spacings.

The 500 mm mesh has produced a higher peak moment; this is

due to singularities or infinite stresses and internal forces that occur

at the location of high point loads. This is due to assumptions that have

been made in the model. In flat slabs the concrete will crack and the

reinforcement yield locally and thus distribute the forces to adjacent areas.

Definitive advice cannot be given as to the ideal size mesh size, but agood starting point is for elements to be not greater than span/10 or

1000 mm, whichever is the smallest.

For large models it is worth running the initial analysis with a coarse

mesh, which can then be refined when the model has been proved to

be free of errors or warnings and gives reasonable results. With most

software packages the meshing is carried out automatically and the

software can even reduce the element size at critical locations to

obtain more data where it is most needed. This will give more detailed

results without a significant increase in analysis time.

Element shapeElements should be well conditioned, i.e. the ratio of maximum to

minimum length of the sides should not exceed 2 to 1 (See Figure 6).

Again this is because the results are accurately calculated only at the

node positions. It is important to ensure that there are more nodes

included in the model where the forces change rapidly because it is

only at node locations that results are obtained directly; in between

the nodes the results given are based on interpolation.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

Distance (m )

1000 mm mesh (4 min)500 mm mesh (15 min) 5000 mm mesh (30 sec)

640

590

540

490

440

390

340

290

240

190

140

90

40

-10

-60

-110

Bending

moment(kNm

)

Figure Bending moments: accuracy of results compared with mesh size

a) Well conditioned b) Poorly conditioned

Figure Element shape


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SupportsIt is important to correctly model the support conditions to ensure

that resulting bending moments at the supports and in the mid-span

are realistic. It will also enable column moments to be derived and

punching shear stress to be realistically evaluated. Where bending isinduced in the columns, i.e. for a monolithic frame, the stiffness of

the column should be modelled; this is particularly true for edge and

corner columns. Where these columns are modelled with vertical point

supports only, the bending moments at the interior columns and spans

can be underestimated. This can also lead to inaccuracies in the local

forces around the supports.

These potential errors, combined with the potential for deflection

results at mid-span to be increased by 10% when using point supports,

mean that the area of the column should be modelled. This can be

achieved in two ways. Either by inserting a thicker region in the slab to

match the plan area of the column, or by using rigid arms between the

column centreline and its perimeter (see Figure 7). Neither is a perfect

solution, but both are more realistic than a point support.

The stiffness of the columns should be modelled by using rotational

spring stiffness. For a pin-ended column the stiffness can be taken as

K= 3EI/l and for a fully fixed column K = 4EI/l (see Figure 8). However,

for columns supporting the upper storeys, edges and corners the

end condition will not be fully fixed and cracking can occur that will

reduce their stiffness. Further if edge and corner columns are made too

stiff they will attract more moment to them, which may exceed the

maximum transfer moment.

The rules for governing the maximum moment that can be transferred

between the slabs and the column are given in Eurocode 2,

Annex I:1.2.(5) or in BS 8110 Cl. 3.7.4.2 & 3.7.4.3. These rules are

applicable even when using FE analysis. If the maximum transfer

moment is exceeded the design sagging moment should be increased

to reduce the hogging moment at the critical support.

For non-symmetrical columns the stiffness will be different in each

direction. Many modern FE packages will automatically calculate the spring

stiffness, and all the user is required to do is enter the column dimensions.

Other problems with supports can occur at the ends of walls and

where columns are closely spaced. In these situations the results will

show sharp peaks in the bending moments, shear forces and support

reactions (see Figure 9). This is due to singularity (infinite stresses)

problems that occur with linear-elastic models. In reality these peaks

do not exist in the concrete because it will crack and yield. Modelling

this behaviour is difficult using linear elastic behaviour, but one method

is to use vertical spring supports near the ends of walls to spread

the peak support reaction on the end node to adjacent nodes. Some

programs include features designed to deal with this situation.

a) Deep region b) Rigid arms

Figure Alternative methods for modelling the area of the column

Interrupted

support

System

Interruptedsupport

Figure Support forces in interrupted line support

L1L1 L1

L1

L2L2L2

L24EI2 3 EI2

4EI 1 3EI1

a) Far end fixed b) Far end pinned

Figure Modelling column stiffness


10/16

10


LoadingAll software will allow a number of load cases to be considered, and

the engineer must assess how to treat pattern loading. It requires

engineering judgement to determine the most unfavourable

arrangement of design loads for a floor plate with an unusual geometry.However, Eurocode 2 gives some specific guidance in Annex I on how to

deal with loading for unusual layouts.

Where pattern loading is to be considered, the maximum span

moments for flat slabs designed to BS 8110 can be obtained by using

the combination of unfactored dead load over the full length of a

bay alternating with the factored dead and live loading across the full

length of the adjacent bay (see Figure 10, arrangements 2 to 5).

When designing using Eurocode 2 the combination of the full factored

dead load over the whole slab together with the factored live loading

on alternate bays should be used (see Figure 10). These should be

considered separately in each orthogonal direction. Note that a

chequer-board pattern loading is an unlikely pattern and may not give

the most unfavourable arrangements.

The engineer should be aware that problems can occur in the way FE

programs assign forces to the nodes of the elements (see Figure 11).

In Figure 11a), a uniformly distributed load is applied to a beam using finite

elements that are a third of the length of the beam. The software will

determine the load to be applied to each node based on the parametric

functions of the element type being used. In this case the load is

apportioned equally to the node at either end of the element. The analysis

gives an approximation only of the bending moments and shear forces.In Figure 11b) a central point load is analysed as two point loads at

one third distances, which gives incorrect bending moments and shear

forces. Finally in Figure 11c) an upwards load on the middle element of

the beam leads the FE software to calculate there is no load at all on

the beam and hence no forces.

The conclusions to draw are that the mesh needs to be more refined if

patch loads are applied to a model and that a node should always be

placed at the location of a large point load. Some software may apply

a corrective moment where point loads do not coincide with nodes. If

this is the case and the user is relying on this feature, the results should

be validated.

For non-linear cracked section analysis, two stiffness matrices will

be required, one each for the ULS and SLS. This is because the slab

is almost certainly not fully cracked at the SLS and the material

properties will be different from those at the ULS. The loads should be

assigned to both cases with appropriate partial factors.

ValidationAs with any analysis it is necessary to validate the results in order

to avoid errors in the modelling and input of data. There is a risk of

engineers assuming that because the computer can accurately andrapidly carry out complex calculations it must be right. The failure

Load arrangement 1

Load arrangement 5

Load arrangement 2

Load arrangement 3

Load arrangement 4

Key

BS 8110

1.0 Gk

1.4 + 1.6G Qk k

Eurocode 2

gG kG

G k Q k+G Q

( Note is always the same

value throughout slab)

gG

g g

Figure 10Load arrangements for flat slabs


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11


10 kN/m

25 kN

FE nodal forces

Real loading

50 kN 50 kN

15m

25 kN

75 kN

Shear force

Shear force

Bending moment

Bending moment

50 kN

100 kN

50kN

15 m

50 kN 50 kN

75 kN

10 kN/mReal loading

3 x 5 m

Finite element model

FE nodal forces

Member forces = Deflection = 0

FEM

Analytical results

a) Uniformly distributed load

b) Point load

c) Exceptional loading

Key

FE nodal forces

Real loading

0 kN 0 kN25 kN 25 kN

Figure 11Support forces in interrupted line support

of the Sleipner a platform in the North Sea in 1991 is a sobering

reminder of what can happen when it is assumed that the results

from a FE model are correct. As the platform was being lowered into

position one of the cell walls failed, which led to the destruction of the

whole structure. One reason for the failure was that the mesh was too

coarse in a critical location to detect the peak forces. The total financial

cost of the disaster has been calculated as $700M; fortunately there

was no loss of life.

There are number of simple checks of the analysis that must be carried

out and the results of these checks should always be included when

the calculations are presented.

Are the supports correctly modelled?

Is the element size appropriate particularly at locations with

high stress concentrations?

Is there static equilibrium? Calculate by hand the total applied

loads and compare these with the sum of the reactions from the

model results.

Carry out simplified calculations, by making approximations if

necessary. (This could be done by using yield line methods or the

RC spreadsheets11). If the FE results vary from these calculations

by more than 20% the cause will need to be investigated .

Do the contour plots look right? Are the peak deflections and

moments where they would be expected? Sketch out by hand

the expected results before carrying out the analysis.

Is the span-to-effective-depth ratio in line with normal practice

(see Table 1).

These checks should always be carried out before any attempt is made

to design the reinforcement.

The engineer should be confident the software is doing what is

expected. Most solvers have a good track record and can be used with

confidence to obtain analysis results (provided the input data is correct

and assumptions understood). However, the design post-processors are

less tried and tested. The engineer should be satisfied that the design

of the reinforcement, particularly for the deflection calculations, is

being carried out as expected. When new software is being used some

validation against known benchmarks should be carried out.

It would also be of assistance to the practicing engineer if a summary

sheet of assumptions and design methods built into the software were

provided so they can be easily assimilated.

Ultimate limit state designTwisting momentsTreating reinforced concrete as an elastic isotropic material can lead

to problems in interpreting the bending moment results. The output

from an FE analysis of plate elements will give bending moments inthe x and y directions, Mx and My. However, it will also give the local

twisting moment Mxy (see Figure 12). This moment is significant and


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must be considered in the reinforcement design. Mxy does not act

in the direction of the reinforcement and a method is required to

allow for Mxy in the design. A popular method in the UK is known as

Wood Armer moments, although it is not the only method used. Most

software will calculate Wood Armer moments for the user. They have

four components, top (hogging) moments in the x and y directions,

Mx(T) and My(T), and bottom (sagging) moments in each direction,

Mx(B) and My(B). The method is slightly conservative and these

moments form an envelope of the worst-case design moments. It is

possible to have both Mx(T) and Mx(B) moments at the same location

in the slab (usually near the point of zero shear).

The four components can be used directly to calculate the required

reinforcement for each of the four reinforcement layers in a flat slab.

Design moment adjustmentWhere high peak moments occur the concrete will crack and the

reinforcement may yield if its the elastic limit is exceeded. The forces

are then shed to the surrounding areas. Even if a slab were designed

to resist this moment it is unlikely that it would actually achieve this

capacity for the following reasons:

The construction process often leads to construction stage overload.

The reinforcement is unlikely to be placed at exactly the point of

peak moment.

It is therefore necessary to acknowledge that some shedding of

the peak moments to adjacent areas will occur due to the material

properties of concrete, and not attempt to design against it. In fact

a recent paper by Scott and Whittle13 concluded that redistribution

occurs even at the SLS because of the mismatch between the uniform

flexural stiffnesses assumed and the variation in actual stiffness that

occurs because of the variations in the reinforcement.

When using FE, especially for slabs with irregular geometry, it is not

usually possible to carry out redistribution of the moments for the

following reasons:

It is not simple to determine where to distribute the hogging

moment to.

If the software is carrying out the design there is usually no method

for changing the analysis output.

In the future, software that models the yielding of the reinforcement willautomatically redistribute the moments and find an equilibrium solution.

Punching shearAlthough an FE model will produce shear stresses, where the columns are

modelled as pins they have no effective shear perimeter and the shear

force is infinite. In this case the simplest way to check punching shear

is to take the reactions from the model and carry out the checks in the

normal way using the provisions in the codes of practice. This can be

automated by using a spreadsheet for the design of reinforced concrete11.

If the area of the column has been modelled, then realistic shear

stresses can be obtained, but some engineering judgement may be

required in using them because there will be peaks which may exceed

the design limits in the codes.

Some software can undertake the punching shear checks and design of

the reinforcement, and the user should ensure that openings within the

shear perimeter are considered in the software.

Interpreting resultsThe results from an FE analysis will generally be in the form of

contour plots of stresses and forces, although a section through the

contour plots (either bending moment or areas of steel) can usuallybe obtained. These will show very large peaks in bending moment at

the supports. The temptation to provide reinforcement to resist this

peak moment should be avoided. This potential error stems from a

lack of understanding of the assumptions made in the modelling. The

reinforcement in the concrete will yield at the support position and

the moment will be distributed across a larger area; it is not therefore

necessary to design to resist this peak moment. However, a method is

required for distributing this peak moment across a larger area.

BS 8110 and Eurocode 2 deal with the peak in bending moment for

flat slabs by averaging it over the column strip and middle strips(Cl.3.7.2.8, BS 8110 and Annex I, Eurocode 2), with the columns strip

sub-divided into inner and outer areas. This method can be used for

designing reinforcement using the results of an FE analysis. A section

is taken across the bending moment diagram (i.e. in the y direction for

moments in the x direction) at the face of the column (the blue line in

Figure 13). The total bending moment is the area under the blue line

(i.e. the integral), which can be apportioned according to rules given

BS 8110 or Eurocode 2.

If the BS 8110 principles are adopted then the design moments would

be as shown by the red line in Figure 13. Here three-quarters of the

total moment is apportioned to the column strip (which is half the

Mxy

Mx

MyMxy

Mx

MxyMxy

My

X

Y

Figure 1Design bending moments compared with FE output


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even for rigorous calculation methods such as non-linear FE analysis.

The engineer would be well advised to include this caveat when

informing clients, contractors and other designers of predicted

deflections.

Of the influences listed above, the three most critical factors are the

values of tensile strength, elastic modulus and creep; their effects have

been discussed previously.

There are several situations where deflections are critical:

Deflection of the slab perimeter supporting cladding brackets/

fixings on the slab perimeter prior to installation of the cladding.

Deflection of the slab perimeter after installation of the cladding.

Deflection of the slab after erection of the partitions.

Where it affects the appearance.

The designer will have to decide which of these apply to an individualproject. Often the load which affects the critical deflection (e.g. deflection

affecting cladding) is not applied at the same time as the initial

loading; in this case the critical deflection can be calculated as follows:

Critical=

Long-term

Deflection prior to criticaldeflection deflection loading being applied

This is because deflection is related to creep and the deflection due to

a critical loading situation cannot be calculated directly.

The accuracy of the deflection calculation can be refined where the age

of loading can be confidently predicted and the type of aggregates to

be used is known. This is more likely to be the case where the designeris working for a contractor or the contractor is part of the design team.

The time of striking and the time when additional formwork loads

from the slab above are applied will have a major influence on the

deflection. This is because the slab is most likely to crack under these

conditions and this will greatly influence the subsequent stiffness of

the slab. The elastic modulus can be more accurately predicted when

the type of aggregate in the concrete is known, and this is more likely

to be the case when the source of concrete has been determined.

Where the loading sequence is known, the critical loading stage at

which cracking first occurs can be established by calculating K for each

stage where:

K= fctm / (W b)

where

fctm = Tensile strength of the concrete

W = Loads applied at that stage

b = 0.5 for long-term loads

The critical load stage is where K is at its minimum and is usually when

the slab above is cast (i.e. construction stage overload), and the tensile

strength should be calculated for this stage. The creep coefficient can

be determined from Section 7.3 of BS 8110 Part 2, or Annex B ofEurocode 2. The Eurocode 2 creep factor allows for a decrease over

time in effective elastic modulus.

Approaches to deflection calculationThe following methods can be used to carry out serviceability limit

state design. They are listed in order of increasing sophistication:

Span-to-effective-depth ratios compliance with code.

Linear finite element analysis with adjustment of elastic modulus.

Non-linear finite element analysis.

The first method should need no further explanation (guidance is given

in both BS 8110 and Eurocode 2); it is the most popular method for

checking deflection and, where the criteria are met, there is no need to

carry out any further checks unless a predicted deflection is required.

The other methods are discussed below.

Linear FE deflection analysisThe linear finite element method should be used only to confirm that

deflection is not critical and not a tool to estimate deflection. This

method involves calculating the elastic modulus and slab stiffness by

hand and adjusting the parameters used in the analysis. A cracked section

analysis is carried out to determine the stiffness of the slab. The cracked

section properties vary with the reinforcement size and layout, so this is

an iterative process and should ideally be carried out for each element

in the slab. However, for initial sizing it is not unreasonable to assume

that the cracked section stiffness is half the gross section stiffness15, or to

use a cracked section stiffness for a critical area of the slab and apply it

globally, provided that it is not used to estimate deflection.

Changing the slab stiffness in an FE model cannot usually be carriedout directly because most finite element packages calculate section

properties from the thickness of the elements. The overall depth of the

concrete should be used, as this gives the correct torsional constant.

However, to allow for a reduction in slab stiffness, the elastic modulus

can be adjusted by multiplying by the ratio of the cracked to uncracked

slab stiffness, R, to model the correct slab thickness. So an appropriate

long-term elastic modulus is R EST/(1 + h) where EST is the short-term

elastic modulus and h is the creep factor, which can be determined

from Section 7.3 of BS 8110 Part 2, or Annex B of Eurocode 2.

In general, the long-term elastic modulus is usually between a third

(for storage loads) and a half (for residential loads) of the short-termvalue15. Therefore, allowing for the need to adjust for cracked stiffness,

the long-term elastic modulus should be in the range one sixth to a

quarter of the short-term elastic modulus.

It is important to recognise that in following this advice the value used

for elastic modulus is in some ways a fudge. It is modelling, in a single

material property, the effects of creep, cracked section properties and

elastic modulus.

Non-linear FE deflection analysis

When using non-linear software, several analyses will often be requiredto obtain a final result. The software will carry out an iterative analysis


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to determine an initial deflection; this will be based on initial, assumed,

areas of reinforcement.

As discussed previously an important aspect to achieving a realistic

estimate of deflection is to consider the loading history for the slab;

once the slab has cracked (and hence has reduced in stiffness) thiswill affect the deflection throughout the life of the slab. This should be

considered in the model.

The slab may not be cracked everywhere; rather it may be fully cracked

in the zones of maximum moment, and in other places it may be

only partially cracked or not cracked at all. An accurate assessment of

deflection can only be made where the appropriate section properties

are calculated for each element in the slab.

Software giving the most accurate deflection calculations will consider the

shrinkage effects. Shrinkage depends on the water/cement ratio, relative

humidity of the environment and the size and shape of the member. Theeffect of shrinkage in an asymmetrical reinforced section is to induce a

curvature that can lead to significant deflection in shallow members.

Once the initial deflection has been determined, an assessment of

the results should be carried out to decide whether the initial areas of

reinforcement were appropriate. If not, they should be revised and the

analysis re-run until there is convergence. It will be necessary to run the

ULS model again with the correct reinforcement, because varying the area

of reinforcement will alter the slab stiffness and hence the distribution of

the moments (i.e. the stiffness at the supports will be reduced because of

cracking and hence moment will be shed to other areas).

There will be different assumptions built into each piece of software

and so it is very important that the engineer is fully aware of the

assumptions and the effects they will have on the design.

Software design toolsEngineering software is developing all the time, particularly the

tools that are available to assist with design. Increasingly, software

will produce a reinforcement layout based on the analysis and post-processing. The efficiency that can be achieved, especially when late

changes to the design occur, is substantial. It is important for the user

to thoroughly understand the software and the methods employed.

The particular areas to consider are:

How is deflection calculated?

How is the additional reinforcement required for deflection control

calculated and incorporated into the design?

How are the design moments apportioned to column and middle

strips and reinforcement layouts produced?

Is a check on maximum moment transfer to the columns included

or should this be carried out by hand?

SummaryThe use of FE analysis and design is certain to increase in the future.

Currently it is a very useful method for slabs with irregular geometry,

for dealing with openings in the slab and for estimating deflections.

However, it is important to realise that the technique will not give lower

design bending moments for regular grids. Having read this guide the

practising engineer should be able to understand the following issues:

How to correctly model concrete.

How the software works and the difference between the types ofsoftware available.

How to validate the software and the models analysed.

How to interpret the results.

References

1 BRITISH STANDARDS INSTITUTION. BS EN 199211: Eurocode 2: Design of concrete structures. General rules and rules for buildings. BSI, 2004.

BRITISH STANDARDS INSTITUTION. BS 81101: The structural use of concrete Code of practice for design and construction. BSI, 1997.

GASPARINI, D.A. Contributions of C.A.P. Turner to development of reinforced concrete at slabs 1905-1999. Journal of Structural Engineering, 2002,

128, No 10, pp 1243 - 1252.

LONDON COUNTY COUNCIL. London Building Act. LCC,1930.

MELOSH, R J. A stiffness matrix for the analysis of thin plates in bending. Journal of the Aerospace Sciences, Vol. 28, No. 1, Jan 1961. pp 34 42.

JONES, A E K & MORRISON, J. Flat slab design past present and future, Structures and Buildings, April 2005.

GOODCHILD, C H. Economic concrete frame elements. BCA, 1997.

THE CONCRETE SOCIETY. TR58: Deections in concrete slabs and beams. The Concrete Society, 2005.

THE CONCRETE CENTRE. Case studies on applying best practice to in-situ concrete frame buildings . The Concrete Centre, 2004.

10 COLLINS, M R et al. The failure of an offshore platform, Concrete International, August 1997, pp 29 42.

11 GOODCHILD, C & WEBSTER, R. Spreadsheets for concrete design to BS 8110 and Eurocode 2, version 3. The Concrete Centre, due 2006.

1 PALLETT, P. Guide to at slab formwork and falsework. Construct, 2003.

1 SCOTT, R H & WHITTLE, R T. Moment redistribution effects in beams, Magazine of Concrete Research, 1 February 2005, pp 9 20.

1 THE CONCRETE SOCIETY. TR43: Post-tensioned concrete oors design handbook (Second edition). The Concrete Society, 2005.1 WHITTLE, R T. Design of concrete at slabs to BS 8110, Report 110, revised edition. CIRIA, 1994.


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How to design reinforced concrete slabs using finite element analysis Finite Element Analysis

All advice or information from The Concrete Centre is intended for those who will evaluate the significance and limitations of its contentsand take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice orinformation is accepted by The Concrete Centre or its subcontractors, suppliers or advisors. Readers should note that publications from

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analysis, and for assistance relating to the

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Design using FE analysis synopsis

1 FE analysis will not reduce the slab thickness significantly compared with other methods of analysis.

Linear FE analysis is widely used and is more than adequate for many situations.

Non-linear FE analysis is more sophisticated and can be used to estimate deflection.

It is important to carry out hand checks prior to FE analysis.

There should be sufficient nodes in the model to obtain accurate results, but it is possible to have too many nodes, especially at supports

where the peak moments are accentuated. Elements should be smaller than span/10 or 1000 mm.

The element shapes should be well-conditioned. An aspect ratio of less than 2 to 1 is appropriate.

The column stiffness should be modelled, i.e. pinned supports are not recommended.

The area of the column should be modelled i.e. point supports are not recommended.

Pattern loading should be considered, but chequerboard loading is not appropriate.

10 Validate your results by considering:

Element size in critical locations.

Is there static equilibrium?

Do hand checks give similar results?

Do the graphical results look right?

Are the results in line with those for similar structures?

11 Understand the software. Ask for a summary guide from software suppliers.

1 Ensure that twisting moments are considered in the design.

1 Do not design the reinforcement for the peak moments; take an average moment over an appropriate width.

1 Even the most sophisticated deflection analysis will be accurate only to +15% to 30%.

1 With linear FE analysis use an elastic modulus value modified to take account of creep and slab stiffness in order to check deflections are

within limiting criteria.

1 If using non-linear analysis to obtain deflection estimates, it is important to critically appraise the software and understand its limitations.

AcknowledgementsThe content and illustrations have come from many sources. The help and advice received from many individuals are gratefully acknowledged.

Special thanks are due to the following for their time and effort in commenting and providing technical guidance in the development of this

publication:

Kenny Arnott CSC (UK) Ltd

Allan Bommer RAM International

Tony Jones Arup

Des Mairs Whitbybird

John Morrison Buro Happold

Robert Vollum Imperial College, London

Rod Webster Concrete Innovation & Design

CCIP_How to FE Analysis on Slab

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